What Is Alteration?

Minerals Can Change, Even at Low Pressure

Geological Formations
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Alteration is a mineralogical change at low pressures due to invading fluids or the influence of oxygen. Alteration may take place near the Earth's surface, where it is considered part of chemical weathering. This is called supergene alteration. More often, however, alteration occurs deep inside the Earth where super-heated water leads to the development of veins of ore minerals. This is called hydrothermal alteration

Examples of Alteration

Supergene alteration provides geologists with information about the type of ore that may be hidden under the Earth's surface. Chlorargyrite, or horn silver, is the mineral form of silver chloride. It forms as a result of supergene alteration. Its presence suggests that silver ore may be present in the area.

Hydrothermal alteration occurs when very hot water from inside the Earth interacts with minerals near the surface. California's gold deposits can be linked to the activity of hydrothermal alteration. This geological process has helped to shape the history of the United States.